Nanoclusters composed of 2-8 silver atoms could be the basis for a new type of optical data storage. Fluorescent emissions from the clusters could potentially also be used in biological labels and electroluminescent displays.
Writing in the journal Science, researchers at the Georgia Institute of Technology report that they have successfully demonstrated binary optical storage with the new system, writing and reading simple images recorded on thin films made up of silver oxide (Ag2O) nanoparticles.
"These nanomaterials have a remarkable new property: when you shine blue light with a wavelength of less than 520 nanometers onto them, you switch on their ability to fluoresce," said Robert M. Dickson, assistant professor of chemistry and biochemistry at Georgia Tech. "You can then read the fluorescence nondestructively by illuminating the clusters with longer-wavelength light."
The researchers begin by producing extremely thin films (less than 20 nanometers thick) of silver oxide nanoparticles on a glass slide. They then selectively expose portions of the film to light in the blue spectrum. The light chemically reduces particles near the surface of the film, partially converting them to clusters of silver atoms. When researchers then expose these photoactivated silver clusters to longer wavelength (greater than 520 nanometers) green light, they fluoresce strongly, emitting red light easily visible to the naked eye. Silver oxide particles not photoactivated by exposure to the blue light do not fluoresce.
Dickson's research group, including Lynn A. Peyser, Amy E. Vinson and Andrew P. Bartko, have used the technique to store images of simple geometric shapes and the letter "L."
When studied under a microscope, the individual silver particles display an additional property that may ultimately prove useful for increasing the density of optical data storage.
"If you look at an individual particle through the microscope, you see green emission, then red emission, then yellow emission all from the same particle," Dickson said. "Not only are you generating fluorescence, but you presumably are also changing the size and/or geometry of the cluster, which causes it to emit different wavelengths."
By using the correct distribution of particle sizes, these multi-color emissions could allow storage of more than one bit of information in each data point. And if the particles could be distributed in a three-dimensional matrix, they could provide a very dense storage medium that could be written and read in parallel.
"We have already demonstrated binary optical storage because we can write fluorescent patterns in which an individual particle is either on or off," Dickson noted. "But we can imagine being able to write and read more than binary storage. These silver clusters could potentially be very useful optical storage materials because of the potential for writing and reading in parallel, and/or storing more than one bit of information per data point."
When exposed to laser-generated blue light at a wavelength of 515 nanometers, the nanoclusters produce a seemingly random blinking pattern of yellow, red and green light. Exposure to blue light, however, photoactivates additional silver oxide particles, destroying the original image.
Images stored on the silver oxide film can be read nondestructively by green light for at least two days, the longest period of time the researchers left them on the stage of their microscope. How long the effect will persist is a topic for further study.
Though they have demonstrated an ability to optically write and read information with the new system, the researchers do not yet know if the information can be optically erased and the film re-written.
Fluorescence has previously been reported in silver clusters at low temperatures and in rare gas environments, but Dickson believes this is the first time the phenomenon has been reported at room temperature.
Having demonstrated a potentially valuable new technique, the researchers are now working to understand the fundamental issues governing the properties of the nanoclusters.
"We really want to understand the underlying physics and chemistry of this material," Dickson said. "While we have an eye toward developing applications, the issue now is understanding what gives rise to the fluorescence, understanding the size and geometry of these clusters, how to control the composition and what factors are important for generating the fluorescence."
A physical chemist with a background in optically-active organic dyes, Dickson expected to see fluorescence in the silver clusters, but he was surprised at the strength of the emissions produced. "We were also amazed at the beauty of the fluorescence from the sample," he added.
Photoactivation of silver halide crystals has been the basis for commercial photographic processes used for more than 100 years. The new technique is similar, though photographic materials use larger crystals of silver salts as the photoactivable material.
While the researchers do not yet understand why the particles fluoresce, Dickson believes the phenomenon's cause relates to the quantum mechanical properties of atomic silver. "Interesting things happen when materials that behave in one way as bulk materials are reduced to the small scale," he added.
Students involved in this research were supported in part by the Georgia Tech Molecular Design Institute and the National Science Foundation's Research Experiences for Undergraduates program. Provisional patent protection has been applied for to protect the technique.
The paper appeared in the January 5 issue of Science.
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